1. Field of the Invention
The present invention relates to a thermally conductive sheet which promotes the heat dissipation of heat-generating electronic parts and the like.
2. Description of the Related Art
Along with higher performance of electronic devices, the density enhancement and packaging enhancement of semiconductor devices have proceeded. Along therewith, it becomes important to more efficiently dissipate heat generated from electronic parts constituting electronic devices. Semiconductors are attached to heat sinks such as heat-dissipating fans and heat-dissipating plates through thermally conductive sheets in order to efficiently dissipate heat. As the thermally conductive sheets, materials in which filling materials such as inorganic fillers are dispersed and contained in silicones are widely used.
A further improvement in thermal conductivity is required of such heat-dissipating members, and this is generally coped with by increasing the filling rate of the inorganic filler blended in the matrix for the purpose of high thermal conductivity. When the filling rate of the inorganic filler is increased, however, since the flexibility is impaired, and powder falling occurs due to the high filling rate of the inorganic filler, there is a limit to increasing the filling rate of the inorganic filler.
Examples of the inorganic filler include alumina, aluminum nitride, and aluminum hydroxide. Further for the purpose of high thermal conductivity, the matrix is filled with scaly particles of boron nitride, graphite, or the like, carbon fibers, or the like, in some cases. This relies on the anisotropy of the thermal conductivity of the scaly particles or the like. For example, in the case of carbon fibers, they have a thermal conductivity of about 600 W/mK to about 1,200 W/mK in the fiber direction. In the case of boron nitride, it has a thermal conductivity of about 110 W/mK in the plane direction and a thermal conductivity of about 2 W/mK in the direction perpendicular to the plane direction, and is known to have anisotropy.
The direction of the carbon fibers and the plane direction of the scaly particles are thus made to be the same as the thickness direction of the sheet, which is the heat transfer direction. That is, by orienting carbon fibers and scaly particles in the sheet thickness direction, the thermal conduction can be improved remarkably. However, in the case where when a cured product cured after being molded is sliced, the cured product cannot be sliced into a uniform thickness, irregularities of the sheet surface are large and catch in air, posing a problem that the excellent thermal conduction is not made the best use of.
In order to solve the problem, for example, Japanese Patent Application Laid-Open (JP-A) No. 2010-56299 proposes a thermally conductive rubber sheet prepared by stamping out and slicing with blades arranged at the same intervals in the perpendicular direction to the longitudinal direction of the sheet. Further JP-A No. 2010-50240 proposes that a thermally conductive sheet having a predetermined thickness is obtained by slicing, with a cutting device having a round rotating blade, a laminated body prepared by lamination by repeating coating and curing. Further JP-A No. 2009-55021 proposes that a laminated body prepared by laminating two or more layers of graphite layer containing anisotropic graphite particles is cut (at an angle of 90° with respect to the laminated plane) by using a metal saw so that an expandable graphite sheet is oriented at 0° with respect to the thickness direction of the obtained sheet. These proposed cutting methods, however, causes the surface roughness of the cut surface to become large and the thermal resistance at the interface to become high, posing a problem that the thermal conduction in the thickness direction decreases.
In recent years, there are desired thermally conductive sheets to be used by being interposed between various types of heat sources (various types of devices, for example, LSI, CPU, transistors, and LED) and heat-dissipating members. In order that such thermally conductive sheets fill up differences in gap between the various types of heat sources and the heat-dissipating members and cause them to closely adhere with each other, there are desired thermally conductive sheets which are compressible and soft.
Although the thermal conductivity of thermally conductive sheets is usually raised by filling a large amount of thermally conductive inorganic fillers (for example, see JP-A Nos. 2007-277406 and 2007-277405), when the filling amount of the inorganic fillers is made large, the sheets become hard and brittle. Further for example, in the case where silicone-based thermally conductive sheets filled with a large amount of inorganic fillers are placed in a high-temperature environment for a long time, such phenomena occur that the thermally conductive sheets become hard and the thickness become large, and the thermal resistance of the thermally conductive sheets in the load-applied time ends in rising.